This research project involves the definition of the mechanisms concerned in the production of enhancement to normal tissue in the mouse. Several different lines of investigation are currently being followed: (a) A study of the site of action of immunosuppressive alloantisera. (b) A definition of the role of the H-2 complex in the production of enhancement and of the immunogenicity of individual H-2 alloantigens. (c) A study of Ir (immune response) genes which are involved in the production of antiserum which has the potential to enhance.